236 OUTLINES OF CHORDATE DEVELOPMENT 



having degenerated to functionless vestiges; the left ovary and 

 oviduct correspondingly become very large. 



The ovary (Fig. 86) appears to be composed of a mass of 

 globules of varying size, suspended from the dorsal abdominal 

 wall by a double peritoneal fold known as the mesovarium 

 (cf. mesentery). These globules are partly grown, immature 

 ova, and in the adult hen they vary in size from a simple cell up 

 to the full-sized vitellus. The oviduct is a large, rather thick- 

 walled, muscular tube, considerably convoluted and showing 

 a well-marked regional differentiation. Its upper end opens out 

 of the body cavity from the region of the ovary, while its lower 

 end discharges directly into the cloaca. Three general regions 

 are to be distinguished : the first, or oviduct proper, is the most 

 extensive and is itself divisible into three sections. The abdom- 

 inal opening of the oviduct is a wide, flaring, funnel-shaped 

 opening called the ostium, or infundibulum, or sometimes the 

 fimbriated opening, from its fringe-like margin; its walls are thin 

 and muscular, and it is lined internally with cilia. The ostium 

 leads directly into the long convoluted glandular portion, and 

 this is followed by the short third section of the true oviduct, 

 the isthmus. The second general region is the so-called uterus, 

 a dilated portion, also with glandular walls. The short terminal 

 region is a thin- walled vagina, which opens into the cloaca just 

 dorsally to the opening of the rectum. The functional charac- 

 teristics of these regions will be described in the following 

 paragraphs. 



3. The Formation of the Egg and its Early Development 



The early developmental history of the egg may conveniently 

 be described in three periods: (A) ovarian development to 

 the time of ovulation; (B) from ovulation through fertilization; 

 (C) from the beginning of cleavage to the time of laying. 



A. THE HISTORY OF THE OVARIAN OVUM TO OVULATION 



The rhythm of egg production in the domestic fowl is unusual 

 in that, as a rule, a long period of egg formation and laying, 

 extending over several months, is followed by a relatively 



